1. Field of the Invention
The present invention relates to a video display controller for use in terminal equipment for a computer, television game apparatus or the like.
2. Prior Art
Recently, various kinds of display control systems which display animation and still images on a screen of a CRT (cathode-ray tube) display unit under the control of a CPU (central processing unit) have been developed. FIG. 1 shows one example of such conventional systems which comprises a video display controller (hereinafter referred to as "VDP") 101 and a central processing unit (CPU) 102. The system further comprises a memory 103 which includes a ROM (read only memory) for storing a variety of programs to be executed by the CPU 102 and a RAM (random access memory) for storing other necessary data. The CPU 102 outputs data representative of still and animation images to be displayed on a screen of a CRT display unit 105 to the VDP 101 which in turn stores the still and animation data into a video RAM (hereinafter referred to as "VRAM") 104.
Upon receipt of a display command from the CPU 102, the VDP 101 sequentially reads the still and animation data from the VRAM 104 in accordance with scanning synchronization signals of the CRT display unit 105, and supplies the read data to the CRT display unit 105, thereby the still and animation images being displayed on the screen of the CRT display unit 105.
Such a kind of display controller described above is generally provided with a kind of code converter called a color palette circuit. The color palette circuit converts each of color codes (codes for designating colors of display elements which constitute still and animation images on the screen) read from the VRAM into three color data RD (red), GD (green) and BD (blue). These color data RD, GD and BD each composed of about three bits are then converted respectively into analog color signals R, G and B to be supplied to the CRT display unit.
In such a display control system, when the number of display elements or dots on the display screen is increased to improve the quality of display image, the rate of conversion of color codes of the color palette circuit must also be increased. Incidentally, it has become common that such a video display controller incorporating a color palette circuit is constructed as an LSI. And therefore, in order to increase the conversion rate of a color pallete circuit, the following problems must have been solved.
When it is desired to increase the conversion rate, the propagation delay at each circuit component of the color palette circuit portion in the LSI must be reduced by increasing its circuit current. However, to increase the circuit current, the area of the each circuit component must be increased, which results in an increase of circuit areas of the color palette circuit portion in the LSI. In addition, when the circuit current is increased, the power dissipation and generation of heat of the color palette circuit portion of the LSI is increased.